2002 Mitsubishi Magna no brake lights

i have no rear brake lights at all which is the 2 at the rear and the 3rd on the rear window in the center. i have checked bulbs they look fine and i have checked the fuses they look fine too from what i can see. i have taken out the brake light switch there seems to be nothing wrong with that either could it be the relay that feeds the brake light switch power or maybe magna's run off a completely different relay of some sort themselves. im trying to go through the process of elimination and take as much information of the internet as i can and do it myself. i bought this car of a friend for my sister i know they installed a head unit and some speaker above the back seat but im unsure where the interference is coming from for me to have NO stop lights what so ever.
thanks